1. Field of the Invention
The present invention relates to a photostrictive device for use with an optical actuator and more particularly to a photostrictive device whose surface is improved to enhance its response speed.
2. Prior Art
A PLZT ceramics [(Pb,La)(Zr,Ti)O.sub.3 ] (simply referred to as PLZT) is a ferrodielectric ceramics having a photostrictive effect whereby it extends when it absorbs light, and can convert optical energy directly to mechanical energy.
Recent years have seen major research efforts being made for the development of micromachines. Because it is difficult to use a lead wire to supply electric energy to an actuator, the driving source of the micromachines, it is desired that the energy be supplied remotely to the micromachine actuator without a physical contact.
Hence, expectations are growing that the PLZT, which is a photostrictive device, may be used as an actuator of micromachines by irradiating light against the PLZT to supply energy to the PLZT without physical contact.
The PLZT, when used as the photostrictive device, however, has a response many orders of magnitude slower than when it is normally used as a piezoelectric device that is applied a voltage. This is a major problem that should be addressed before the PLZT can be used as an optical actuator.
FIG. 5 shows the result of experiments conducted by the inventors, in which a specimen of PLZT photostrictive device was irradiated with a specified wavelength of light and in which elongation, current and voltage produced were measured by an electric micrometer, electrometer and electrostatic meter. The elongation of PLZT with the light energy density Ed taken as a parameter changes as shown in FIG. 6(a).
As shown in FIG. 4, the PLZT, upon absorbing light, immediately generates current and voltage and elongates, but the response of elongation is far slower than those of current and voltage.
FIG. 6(b) indicates that although the generated current increases with the light energy density, improving the response of elongation, it saturates at a certain level.
Judging from the above, the elongation of PLZT resulting from absorption of light is a reverse piezoelectric effect caused by the potential difference between the electrode ends. That is, the PLZT elongation is considered to be caused by electrons excited by light absorption moving in the polarization directions.
It is therefore concluded that because the number of electrons excited is limited by the amount of light absorbed by the PLZT, there is a limit to the improvement of response of the PLZT when the response is to be improved simply by increasing the energy density of light to be irradiated.